MEMS sensor structure and microfabrication process therefor

ABSTRACT

A micro-electro-mechanical structure including a semiconductor layer mounted to an annular support structure via an isolation layer wherein the semiconductor layer is micromachined to form a suspended body having a plurality of suspension projections extending from the body to the rim and groups of integral projections extending toward but spaced from the rim between said suspension projections. Each projection in said groups has a base attached to the body and a tip proximate the rim. The structure includes a plurality of inward projections extending from and supported on the rim and toward the body. Each such projection has a base attached to the rim and a tip proximate the body; wherein the grouped projections and the inward projections are arranged in an interdigitated fashion to define a plurality of proximate projection pairs independent of the suspension elements such that a primary capacitive gap is defined between the projections of each projection pair. Also, a process is disclosed for fabricating the micro-electro-mechanical structure including the steps of removing a highly doped etch termination layer and thereafter etching through a lightly doped epitaxial layer to thereby define and release the structure.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application discloses subject matter which is disclosed andclaimed in co-pending U.S. application Ser. No. ______, Attorney DocketNo. DP-300151, filed ______, 1999, in the name(s) of and John CarlChristenson et al., and entitled “Method and Apparatus for ElectricallyTesting and Characterizing Formation of Microelectronic Features,” theentire contents of which are incorporated herein by reference. It isalso related to the co-pending application Attorney's Docket No.H-203587, “Angular Accelerometer,” filed ______, 1999, in the name ofDavid Boyd Rich.

TECHNICAL FIELD

[0002] The present invention relates to micro-electro-mechanical systems(MEMS) and in particular to an accelerometer and relatedmicrofabrication processes for the high-volume manufacture of such adevice.

BACKGROUND OF THE INVENTION

[0003] Presently, micro-structure devices called MEMS(micro-electro-mechanical systems) are gaining popularity in themicroelectronics industry. Such MEMS devices include, for example,micro-mechanical filters, pressure micro-sensors, micro-gyroscopes,micro-resonators, actuators, rate sensors, and acceleration sensors.These MEMS devices are created by microfabrication processes andtechniques sometimes referred to as micromachining. These processesinvolve the formation of discrete shapes in a layer of semiconductormaterial by trenching into the layer with an etch medium. Because MEMStypically require movement of one or more of the formed shapes relativeto others, the trenching is done in part over a cavity and in part overa substrate or bonding layer.

[0004] MEMS technology can be used to form rotary accelerometers. Themain structure of a typical MEMS rotary accelerometer comprises a proofmass supported by a flexure suspension that is compliant for rotationbut stiff for translation. In a known device, the suspension comprisesfingers extending radially from the body straddled by inwardlyprojecting capacitor plates mechanically grounded to surrounding annularsubstrate area; see U.S. Pat. No. 5,251,484, “ROTATIONAL ACCELEROMETER”issued Oct. 12, 1993 to M. D. Mastache and assigned to Hewlett-PackardCo. of Palo Alto.

[0005] Forming the body mass and micro-mechanical parts of the MEMSdevice can generally be accomplished, for example, by a process ofanisotropically etching through one or more upper layers ofsemiconductor material(s) which are situated above a cavity previouslyetched into a lower semiconductor substrate. Such a process for formingthe body mass and micro-mechanical suspension parts of a MEMS device isoften referred to as a “bond/etch-back” process. Other processes,however, can instead be utilized to form and/or release the body massand micro-mechanical parts of a MEMS device. Such other processes caninclude a through-the-wafer etch process; a lateral release etch(confined or isotropic) process; or a lateral selective undercut etch ofa buried layer, a film, or a buried etch-stop layer after a MEMSdelineation etch has been performed.

[0006] In addition to properly forming the main structures of the MEMSaccelerometer, electrically conductive lines are typically integratedwith the structure to provide electrical communication between thestructure and other microelectronic circuits. See FIG. 1 of the Mastachepatent identified above. Furthermore, such a device is typicallyencapsulated and hermetically sealed within a microshell (i.e., a cap).The microshell serves many purposes, some of which include, for example,shielding the micro-mechanical parts of the MEMS device from particle(such as dust) contamination, shielding the micro-mechanical parts fromcorrosive environments, shielding the MEMS device from humidity(stiction) and H₂O (in either the liquid or vapor phase), shielding theMEMS structure from mechanical damage (such as abrasion), andaccommodating the need for the MEMS device to operate in a vacuum, at aparticular pressure, or in a particular liquid or gas (such as, forexample, dry nitrogen) environment.

[0007] A typical MEMS device has a size on the order of less than 10⁻³meter, and may have feature sizes of 10⁻⁶ to 10⁻³ meter. This poses achallenge to the structural design and microfabrication processesassociated with these small-scale, intricate and precise devices in viewof the desire to have fabrication repeatability, fast throughput times,and high product yields from high-volume manufacturing. However, theachievement of these goals often primarily depends upon the ability tosuccessfully execute the critical etching process step in accordancewith a desired predetermined shape of the body mass and themicromechanical parts of a proposed MEMS device.

[0008] MEMS devices such as rotary accelerometers having opposingprojections (fingers) which are interdigitated can present a challengein the microfabrication processes particularly where dimensionallydifferent but equally critical gap spacings must be etched at the sametime. This is a result of the fact that wider gaps typically etch fasterthan narrower gaps.

[0009] There is a need in the art for an improved structural design fora MEMS device having interdigitated elements such as projections whichwill reduce or eliminate the adverse effects associated with the etchprocess. There is also a need in the art for an improved implementationof the etch process which can be utilized to specifically fabricate theabove-mentioned improved structural design for a MEMS device havingopposing, interposed and interspaced projections which will circumventand thereby negate the adverse effects associated with the etch process.

SUMMARY OF THE INVENTION

[0010] The present invention provides a micro-electro-mechanical sensorstructure with an improved design comprising rigid interdigitatedprojections forming capacitive plate elements and, in a preferredembodiment, flexible projections forming a rotationally compliantsuspension. According to the invention, the micro-electro-mechanicalstructure basically comprises a semi-conductor layer which ismicromachined to define a proof mass suspended relative to a supportsubstrate by one or more flexible suspension projections extending fromthe proof mass to a substrate-based support area. Between thesesuspension projections and also extending outwardly from the proof massare sets of additional rigid, spaced apart projections which move withthe proof mass according to a compliance mode established by thesuspension elements, e.g., at right angles to the longitudinal axes ofthe finger-like projections. Interdigitated with such projections arecomplemental projections extending from the support area toward theproof mass and defining, in combination with the rigid body projections,narrow sensor gaps of uniform width and larger, parasitic capacitivegaps. The sensor gaps are formed to exhibit essentially constant gapwidths such that the etch process is easily geared to their formationwith no loss of accuracy due to different etch rates in other areas ofthe film.

[0011] In the illustrative embodiment, the proof mass is generallycircular and the suspension elements and interdigitated capacitanceelements are radially arranged. The compliance mode in this embodimentis circular or rotary. However, linear devices using the principleshereafter explained are readily designed.

[0012] The present invention further provides an improved process forfabricating the micro-electro-mechanical structure with its improveddesign for opposing, interdigitated projections consistent with generalbond/etch-back methods of fabrication. The process basically includesthe steps of providing a first substrate, etching a cavity within thefirst substrate, and forming an isolation layer on the first substrate.Further steps include providing a second substrate, doping the topportion of the second substrate to thereby form an etch terminationlayer, forming a doped epitaxial layer on the etch termination layerportion of the second substrate such that the etch termination layerportion of the second substrate has a higher doping concentration thanthe epitaxial layer. Then, the second substrate is bonded to the firstsubstrate such that the epitaxial layer covers the cavity and is bondedto the isolation layer at the periphery of the cavity of the firstsubstrate. Then, the non-termination layer portion of the secondsubstrate is removed from the etch termination layer portion of thesecond substrate, and the etch termination layer portion of the secondsubstrate is removed from the epitaxial layer. A photoresist is thenapplied on the epitaxial layer, and the photoresist is patternedaccording to a predetermined shape of the micro-electro-mechanicalstructure. Thereafter, a step of anisotropically etching throughsections of the epitaxial layer, as revealed by the patternedphotoresist, is performed to thereby define and release themicro-electro-mechanical structure above the cavity. The remainingpatterned photoresist is then removed.

[0013] According to a preferred process of the present invention, thestep of doping the top portion of the second substrate to thereby forman etch termination layer preferably includes the step of doping the topportion of the second substrate with a p-type dopant comprising boronand germanium. In addition, the step of forming a doped epitaxial layerpreferably includes the step of doping the layer with a p-type dopant.Furthermore, the first substrate and the second substrate preferablycomprise silicon, and the isolation layer preferably comprises silicondioxide.

[0014] Also according to the preferred process of the present invention,the step of applying photoresist on the epitaxial layer includes thestep of utilizing a positive photoresist. In addition, the step ofanisotropically etching through the epitaxial layer to define andrelease the micro-electro-mechanical structure above the cavitypreferably includes the step of contacting the epitaxial layer with aplasma comprising sulfur hexafluoride and oxygen, and the step ofcooling the epitaxial layer to a cryogenic temperature of less thanabout 173 EK.

[0015] Further, according to the preferred process of the presentinvention, the step of patterning the photoresist according to apredetermined shape preferably includes the steps of determining aminimum capacitive gap between the interdigitated projections of themicro-electro-mechanical structure which are nearest to each other,defining the predetermined shape such that each base of each projectionis proximate to at least one tip of another projection by a distancesubstantially equal to the minimum capacitive gap, and selectivelyremoving the photoresist to reveal bare sections of the epitaxial layeraccording to the predetermined shape.

[0016] Other advantages, structural and process design considerations,and applications of the present invention will become apparent to thoseskilled in the art when the detailed description of the best modecontemplated for practicing the invention, as set forth hereinbelow, isread in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention will now be described, by way of example,with reference to the following drawings.

[0018]FIG. 1 is a top view of a sensing element for a rotationalaccelerometer MEMS device;

[0019] FIGS. 2(A) through 2(D) are cross-sectional views of thestructure illustrated in FIG. 1 along section lines A-A′, B-B′, C-C′ andD-D′, respectively;

[0020]FIG. 3 is a partial top view of the structure illustrated in FIG.1, particularly highlighting the cantilevers; and

[0021] FIGS. 4(A) through 4(M) illustrate the primary steps and stagesof the preferred process for fabrication of a MEMS structure havingopposing, interposed and interspaced projections according to thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022] Referring to FIGS. 1 and 2, a rotary accelerometer sensor 40comprises a generally circular, semiconductor mass 10 suspended relativeto an annular support layer 54 by four equally spaced radiallyextending, flexible suspension projections 18. The projections extendinto the body 10, are of relatively thin section, terminate inlarge-area tabs 19 and provide both rotary compliance and translationalstiffness. Projections 18 form the suspension system for body 10 and donot, for all practical purposes, affect the capacitance as hereinafterexplained.

[0023] Between each suspension projection 18, body 10 is formed todefine a group of equally spaced and essentially constant widthcapacitive projections 20 which are integral with body 10 but extendradially outwardly therefrom. Projections 20 have rounded tips 24 whichlengthen the sensor gap as hereinafter explained. The projections 20effectively form one of two opposed capacitor plates as hereinafterexplained. The suspension projections 18, for all practical purposes, donot form capacitor elements.

[0024] The FIG. 1 structure further comprises a four-piece rim structurecollectively defining the second capacitor plate. The rim structurecomprises four identical quadrants each including a rim element 16having opposite end areas adjacent but spaced from a tab 19 andtapering, triangular, inwardly-projecting capacitive projections 30having wide base areas 32 and rounded tips 34. Each projection 20 hasone side lying adjacent and in closely and uniformly spaced relationshipto a complemental side of a projection 30 to form a primary capacitivegap. Moreover, the rim structure is etched such that the capacitive gapcontinues around the tips 24 and 34 to define an S-shape. The othersides of the projections 20 and 30 are more widely spaced from eachother; i.e., two or three times the spacing of the primary gap, togreatly reduce the capacitive coupling therebetween. The circular body10 is widely spaced from rim 16 so as to reduce capacitive coupling inthe radial direction except at the tips of the projections.

[0025] The result is a thin-film structure in which the four rimquadrants with their projections 30 can be electrically connected toform one plate of a capacitor while the body 10 with its projections 20form the other plate. Complementary external electrical connections andcomponents may be as disclosed in the Mastache patent, the disclosure ofwhich is incorporated herein by reference. When subjected to rotationalacceleration about the center axis of the proof mass, the suspensionprojections or tethers 18 flex to permit angular movement of the proofmass and the outwardly extending fingers 20 relative to the rimstructure of the inwardly extending fingers 30. This produces capacitivechanges due to spacing variations in the primary gaps. The suspensionelements 18 function essentially solely in a mechanical support, flexuresuspension role and do not materially contribute to output signalquality.

[0026] The structure of FIGS. 1 and 2 incorporates two structural designadvantages for a MEMS device having opposing interposed and interspacedprojections which will circumvent the adverse effects associated withthe etch lag phenomenon.

[0027] Concerning the first design advantage, the projections 20,according to the present invention, are relatively uniform in widthalong their lengths and have a high length to width ratio. Theprojections 30, on the other hand, are pyramidic in shape such thattheir sides are not parallel. As a result, however, one side of thefirst projection 20 and one side of the second projection 30 of eachprojection pair is substantially uniformly spaced apart from each other,along the length of the first projection 20, by a distance substantiallyequal to the minimum capacitive gap. Projections 20 and 30 can, ofcourse, have various shapes, e.g., straight, angled and curved, so longas the sensor gaps between them are of uniform width. These projections20 and 30, having such desirable dimensions and features, are preparedby the method of the invention which avoids the over-etching associatedwith the etch lag phenomenon along wide trenches. As a result, theelectrical characteristics (such as resistance and capacitance levels)inherent in the thicker and wider structure of each projection (finger)are at desired levels and are no longer adversely affected due to excessthinning of each projection due to the over-etching associated with theetch lag phenomenon.

[0028] As a second design advantage, the tips 24 of the projections 20,according to the present invention, are proximate to the rim 16. Inparticular, the tips 24 are spaced from the rim 16 by a distancesubstantially equal to the capacitive gaps 36. The rim 16 is preferablyshaped such that the circumference of each tip 24 of each firstprojection 20 is substantially uniformly spaced from the rim 16 by adistance substantially equal to the primary capacitive gap. Suchrelatively close spacing between the tips 24 and the rim 16 is madepossible by the method of the invention which avoids the over-etchingassociated with the etch lag phenomenon. The method of the inventionavoids the tendency of prior art methods to excessively etch away tips24.

[0029] Advantageously, the tips 34 of the projections 30, according tothe present invention, are proximate to the body mass 10 to circumventthe effects associated with etch lag. In particular, the tips 34 arespaced from the body mass 10 by a distance substantially equal to thecapacitive gaps 36. Such a close spacing between the tips 34 and thebody mass 10 ensures that over-etching associated with the etch lagphenomenon will neither excessively etch away the tips 34 norexcessively etch into the body mass 10. The body mass 10 is preferablyshaped such that the circumference of each tip 34 of each secondprojection 30 is substantially uniformly spaced from the body mass 10 bya distance substantially equal to the primary capacitive gap.

[0030] Furthermore, concerning the second design advantage, the naturalstructural consequence of the closer spacings between the tips 24 andthe rim 16 is that at least one side of each of the bases 32 associatedwith the projections 30 is proximately located near one of the tips 24by a distance substantially equal to the capacitive gap 36. Such acloser spacing ensures that over-etching associated with the etch lagphenomenon will neither excessively etch into each of the bases 32 norexcessively etch away the tips 24.

[0031] Likewise, the natural structural consequence of the closerspacings between the tips 34 and the body mass 10 is that at least oneside of each of the bases 22 associated with the projections 20 isproximately located near one of the tips 34 by a distance substantiallyequal to the capacitive gap 36. Such a closer spacing ensures thatover-etching associated with the etch lag phenomenon will neitherexcessively etch into each of the bases 22 nor excessively etch away thetips 34.

[0032] Ultimately, as a result of the preferred structure in FIG. 1, thebases 22 of the projections 20 and the bases 32 of the projections 30 nolonger have the tendency to be extraordinarily thin and fragile due tothe etch lag phenomenon. Thus, the preferred structure according to thepresent invention helps eliminate the possibility that the projections20 and the projections 30 may break off.

[0033] FIGS. 2(A) through 2(D) are cross-sectional views of thestructure illustrated in FIG. 1 positioned over a cavity 52 in asubstrate 50. An isolation layer 54 covers the substrate 50 as well asthe lining of the cavity 52. The semiconductor layer 14 is mounted onthe isolation layer 54 at the periphery of the cavity 52 such that thebody mass 10 is suspended above the cavity 52 via cantilever suspensionprojections 18. Capacitive gaps 36 between the tips 24 of the firstprojections 20 and the rim 16, and capacitive gaps 36 between the tips34 of the second projections and the body 10 are particularlyhighlighted in FIGS. 2(A) and 2(B). The cantilevers 18 are attached tothe body mass 10 at points 76.

[0034] The first projections 20 and the second projections 30 aredefined in the semiconductor layer 14.

[0035]FIG. 3 is a partial top view of the structure illustrated in FIG.1, particularly highlighting the cantilever 18. According to thepreferred embodiment of the present invention, the sense structure 40has at least one cantilever 18 connected between the body 10 and the rim16. Each cantilever 18 thereby flexibly mounts the body 10 to the rim 16such that the body 10 along with the rigid projections 20 are capable ofrotational movement relative to the fixed surrounding structureincluding the projections 30 extending from the rim 16. The ideal gapsurrounding the suspension projections 18 is greater than the minimum(sensor) gap between projections 20, 30 and equal to or smaller than theparasitic gap. Each cantilever 18, the semiconductor layer 14, the body10, the first projections 20, and the second projections 30 arecomprised of an electrically conductive, doped semiconductor materialsuch that the differential capacitance between the first projections 20and the second projections 30 can be electrically measured whenever theMEMS sense structure 40 experiences rotational acceleration caused by anexternal stimulus.

[0036] It is to be understood that the particular sense structure 40 foruse in a capacitive rotational accelerometer, as illustrated in thefigures, is only one of many different possible MEMS structures that canincorporate and benefit from the teachings of the present invention. Ingeneral, the novel aspects of the present invention can be utilized andincorporated in other MEMS structures having interdigitated projectionsas well.

[0037] The structure of a MEMS device may generally be fabricated by abond/etch back technique. According to a past implementation of thistechnique, a first semiconductor substrate is formed and a cavity isthereafter etched into this first substrate. Next, an oxidation step iscarried out to thereby form an oxide layer (that is, an isolation layer)over the surface and cavity of the first substrate. In addition to thisfirst substrate, a second semiconductor substrate is formed separatelyfrom the first substrate. The top portion of this second substrate istypically very highly doped (that is, is highly concentrated) withp-type impurities, such as boron and/or germanium, to thereby create anetch termination layer (also sometimes referred to as an etch stop layeror a barrier layer). From a semiconductor fabrication and processingstandpoint, attempted etching with an ICP DRIE (inductively coupledplasma deep reactive ion etch) machine through such a termination layercomprised of highly p-doped silicon, for example, is greatly attenuated.Next, a lightly doped epitaxial semiconductor layer (sometimes referredto as an “epi-layer”) is grown on top of the second substrate. Thisepitaxial layer is to be the layer from which the structure of the MEMSdevice is ultimately defined and released.

[0038] Further regarding the past implementation of the bond/etch backtechnique, once the epitaxial layer is properly formed on the secondsubstrate, the second substrate along with its epitaxial layer is theninverted and bonded over the cavity in the first substrate such that theepitaxial layer covers the cavity and is bonded to the oxide layer (thatis, isolation layer) at the periphery of the cavity. In this invertedconfiguration, the epitaxial layer is thus situated directly above thecavity, and the highly p-doped portion (that is, the etch terminationlayer portion) of the second substrate is on top of the epitaxial layer.After bonding and etch back is completed, an etch process step is thentypically attempted to precisely etch deep trenches through both thehighly p-doped portion (the etch termination layer portion) of thesecond substrate and the epitaxial layer until the cavity underneaththese layers is breached. In this way, the remaining portions of theetch termination layer portion of the second substrate and the remainingportions of the epitaxial layer are together released and suspendedabove the cavity. These remaining unetched portions will then serve asthe micro-machined structure of a MEMS device.

[0039] A significant problem with the particular bond/etch backtechnique described above is that attempting to etch through both thehighly p-doped portion (that is, the etch termination layer) of thesecond substrate and the lightly doped epitaxial layer simultaneously,during the same etching process step, often produces very poor anduneven sidewall profiles in the trenches being etched through these twolayers. This is especially the case for the sidewalls of the trenchesetched into the epitaxial layer. In particular, once the highly p-dopedportion of the second substrate is etched through, the sidewall profilesof the trenches etched into the epitaxial layer are typically notanisotropic in form. That is, the sidewalls of the trenches are notsubstantially vertical and smooth, but are instead heavily striated orsomewhat isotropic in form with undesired lateral etching into thesidewalls of the trenches. Additionally, the silicon (for example) ofthe epitaxial layer may be undesirably micro-masked as a result of theetch termination layer being incompletely etched, thereby undesirablycausing silicon “spires” or “grass” to be formed on the sidewalls andbottoms within the epitaxial layer trenches. Such uneven etching throughthe epitaxial layer is most likely attributable to the disparity in theetch rates inherent in the highly p-doped portion (that is, the etchtermination layer portion) of the second substrate and the lightly dopedepitaxial layer. Of most concern, however, is that such lateral etchinginto the sidewalls of the trenches formed in the epitaxial layerultimately produces a MEMS device structure which is malformed andrendered unfit for customer use. For instance, any silicon “spires” or“grass” undesirably formed within the trenches of the epitaxial layeroften become particulates when the cavity underneath the epitaxial layeris breached during etching. These particulates can prevent or interferewith rotational translation of the suspended body, thereby directlyhindering or preventing proper operation of the MEMS structure.Furthermore, these particulates can also undesirably physically bridgethe gaps between the “capacitor plates” of the first projections and thesecond projections, thereby electrically shorting the first projectionsand the second projections together and rendering the MEMS structureuseless. Thus, as a result, utilization of the particular techniquedescribed above can produce a relatively low product yield. The methodaccording to the present invention significantly improves upon pastimplementations of the bond/etch back technique and produces anisotropicetching (that is, vertical and smooth trench sidewalls) through theepitaxial layer from which a MEMS structure is to be formed. FIGS. 4(A)through 4(M) illustrate the primary steps and stages of the preferredmethod/process for fabrication of the preferred MEMS structure accordingto the present invention.

[0040] As illustrated in FIG. 4(A), a first substrate 50 made fromsemiconductor material(s) is initially formed and provided. According tothe preferred embodiment of the present invention, the first substrate50 is made primarily of silicon. However, the first substrate 50 caninstead be comprised with other materials as well, such as, for example,glass, ceramic, sapphire, and stainless steel. Furthermore, the firstsubstrate 50 can be doped (or not doped at all) with either n-type orp-type impurities at any doping concentration level. This firstsubstrate can be formed by any acceptable method known in the art.

[0041] As illustrated in FIG. 4(B), a cavity 52 is then etched into thefirst substrate 50. The cavity 52 can be formed by any knownconventional means, such as by a wet (chemical) etching technique or bya dry etching technique.

[0042] As illustrated in FIG. 4(C), an isolation layer 54, preferablycomprised of silicon dioxide (commonly referred to as an “oxide layer”),is formed on the top surface of the first substrate 50 and the cavity52. Any conventional means known in the art can be used to grow theisolation layer 54. One approach is to heat the first substrate 50 to ahigh temperature, for example, 850 to 1200 EC, in a controlledatmosphere containing either pure oxygen or water vapor. At such hightemperatures, the oxygen and/or water vapor diffuse into and react withthe silicon of the first substrate 50, thereby forming the silicondioxide layer 54 on the exposed top surface of the first substrate 50.This silicon dioxide layer 54 serves as a bonding oxide, as ahigh-quality electrical insulator, and also as an etch terminationlayer.

[0043] As illustrated in FIG. 4(D), a second substrate 58 is formed andprovided from semiconductor material(s), separately from the firstsubstrate 50. According to the preferred embodiment of the presentinvention, this second substrate 58 is made primarily of silicon. Thissecond substrate 58 can be formed by any acceptable method known in theart.

[0044] As illustrated in FIG. 4(E), the top portion 58A of the secondsubstrate 58 is then doped with a high concentration of either n-type orp-type impurities to thereby transform the top portion 58A of the secondsubstrate 58 into an etch termination layer. The lower portion 58B (thatis, the non-termination layer portion) of the second substrate 58 ispreferably left undoped. However, it is to be understood that the lowerportion 58B may alternatively be doped. In such a case, the lowerportion 58B must have a lower doping concentration than the top portion58A to ensure that the top portion 58A can function as an etchtermination layer. According to the preferred embodiment of the presentinvention, the top portion 58A of the second substrate 58 is preferablydoped with p-type impurities, including boron and germanium. These twop-type impurities can be introduced into the top portion 58A of thesecond substrate 58 via a diffusion technique, via ion implantation, or,preferably, via in-situ doping during epitaxial silicon growth.

[0045] As illustrated in FIG. 4(F), a single-crystal epitaxial layer 60,preferably comprising silicon and lightly doped with a low concentrationof either an n-type impurity or a p-type impurity, is grown on the etchtermination layer portion 58A of the second substrate 58. Preferably,however, this epitaxial layer 60 is lightly doped with a p-typeimpurity. This epitaxial layer 60 is formed by conventional means,preferably by a chemical vapor deposition process used to depositadditional silicon on the etch termination layer portion 58A of thesecond substrate 58 and growing (that is, forming) a single-crystalsilicon epitaxial layer 60 from the vapor phase (commonly referred to as“vapor-phase epitaxy”) on the etch termination layer 58A. Thissingle-crystal silicon epitaxial layer 60 can be doped with n-typeimpurities (such as phosphorus or arsenic) or, preferably, p-typeimpurities (such as, for example, boron or germanium) during the growthprocess by adding the impurities to the gas used during deposition ofthe additional silicon. The significance of having the epitaxial layer60 lightly doped, with either an n-type impurity or a p-type impurity,as compared to the highly doped etch termination layer 58A, is discussedlater hereinbelow.

[0046] As illustrated in FIG. 4(G), once the epitaxial layer 60 isproperly formed on the etch termination layer 58A of the secondsubstrate 58, the second substrate 58 (with the etch termination layer58A) along with the epitaxial layer 60 is then inverted and fusionbonded over the cavity 52 in the first substrate 50 such that theepitaxial layer 60 covers the cavity 50 and is bonded to the isolationlayer 54 (preferably comprised of silicon dioxide) at the periphery ofthe cavity 52. In this inverted configuration, the epitaxial layer 60 isthus situated directly above the cavity 52, and the etch terminationlayer 58A is on top of the epitaxial layer 60.

[0047] In light of the observed problems of undesired lateral etchinginto the trench sidewalls and of the undesired formation of silicon“grass” in the trenches when attempting to anisotropically etch throughboth a heavily p-doped etch termination layer and a lightly dopedepitaxial layer during a single etch process step (as alluded to earlierhereinabove), the non-termination layer portion 58B of the secondsubstrate and then the heavily p-doped etch termination layer 58A of thesecond substrate 58 are both first stripped away and entirely removed,as illustrated in FIGS. 4(H) and 4(I) according to the presentinvention, before etching through the epitaxial layer 60 is performed.This entire removal of the etch termination layer 58A before etchingthrough the epitaxial layer 60 helps to thereafter facilitate clean,anisotropic etching through the epitaxial layer 60 so that the sidewallsof the trenches etched into the epitaxial layer 60 are substantiallyvertical without significant lateral etching into the sidewalls, withoutsignificant vertical sidewall striations, and without the formation ofsilicon “grass.” Furthermore, the fact that the epitaxial layer 60 islightly doped, as compared to the highly doped etch termination layer58A, helps facilitate the clean and highly selective removal of the etchtermination layer 58A from the epitaxial layer 60. As a result of suchselective removal, the thickness of the remaining epitaxial layer 60tends to be more uniform. Such uniformity in the thickness of theepitaxial layer 60 helps ensure that trenches which are etched into theepitaxial layer 60 will breach the cavity 52 underneath the epitaxiallayer 60 within a more predictable period of time. Thus, uniformity inthe thickness of the epitaxial layer 60 enables process engineers, forexample, to monitor the anticipated time periods for etching entirelythrough the epitaxial layer 60 to thereby avoid problems of over-etchingand/or under-etching the epitaxial layer 60.

[0048] Once the etch termination layer 58A is removed from the epitaxiallayer 60, a layer of light-sensitive photoresist 62 is applied over theepitaxial layer 60 as illustrated in FIG. 4(J). According to thepreferred embodiment of the present invention, a positive type ofphotoresist should preferably be utilized instead of a negative type ofphotoresist. Positive photoresist facilitates better process control inthis small-geometry structure. Application and formation of thephotoresist layer 62 can typically be carried out by any knownconventional means. This includes, for example, initially applyingphotoresist, in liquid form, over the epitaxial layer 60. After initialapplication, the bottom of the first substrate 50 is situated on avacuum chuck and then spun at a high rate of speed to produce a thinlayer of photoresist over the epitaxial layer 60. After being spun, thephotoresist is then dried (sometimes referred to as “soft baking” or“pre-baking”) to improve adhesion of the photoresist layer 62 to theepitaxial layer 60.

[0049] Once the layer of photoresist 62 is formed on the epitaxial layer60, the photoresist layer 62 is thereafter patterned according to adesired, predetermined shape of the structure of a MEMS device beingfabricated, as illustrated in FIG. 4(K). Patterning a layer of positivephotoresist by conventional means typically involves the steps of,first, aligning a photomask over the photoresist layer. The photomask ispre-patterned according to the desired, predetermined shape of the MEMSstructure to be fabricated. Next, some areas of the photoresist layerare selectively exposed to high-intensity ultraviolet light which isshown through the pre-patterned photomask onto the photoresist layer.Then, only the exposed areas of the photoresist layer are washed away,so that the sections of the epitaxial layer 60 which are still coveredwith remaining photoresist are protected from being etched away duringsubsequent etching of the epitaxial layer 60. On the other hand, baresections 64 of the epitaxial layer 60 which are no longer covered byphotoresist are then ready to be etched away.

[0050] Once the photoresist layer 62 is patterned, trenches 66 are thenanisotropically etched into the bare sections 64 of the epitaxial layer60 until the cavity 52 underneath the epitaxial layer 60 is breached, asillustrated in FIG. 4(L). In this way, a micromachined MEMS structure 70is defined in the epitaxial layer 60 substantially within a single planeand is released above the cavity 52. (Cantilevers which support andsuspend the MEMS structure 70 above the cavity 52 are not shown in FIG.4(L)). The isolation layer 54 (preferably comprised of silicon dioxide)which lines the floor of the cavity 52 serves as an etch terminationlayer to prevent etching into the first substrate 50 once the cavity 52is breached. This same isolation layer 54 also serves to both physicallyand electrically isolate the first substrate 50 from the epitaxial layer60. Thus, aside from perhaps structural support, the first substrate 50plays no role in the electrical functionality of the MEMS structure. Asa result, there is no real necessity for doping the first substrate 50,and process time, process complexity, and overall cost is therebyreduced.

[0051] According to the present invention, any anisotropic etchingtechnique may generally be utilized to etch the trenches 66 into theepitaxial layer 60. However, according to a preferred implementation ofthe present invention, the following high-precision, anisotropic etchingtechnique may be utilized.

[0052] Particularly, the high-precision, anisotropic etching of theepitaxial layer 60 may be accomplished in an ALCATEL Comptech 602E deepsilicon etch system at cryogenic temperatures (that is, temperaturesless than approximately 173 EK) using sulfur hexafluoride (SF₆) andoxygen (O₂) as the etch gases. ALCATEL Comptech is located in Fremont,California and also has facilities in Annecy, France and in Seoul,Korea. Preferred parameter settings for such a system which arenecessary to execute such a high-precision, anisotropic etch are:

[0053] gas 1: SF₆, 250 sccm

[0054] gas 2: O₂, 3 sccm

[0055] power: 700 W

[0056] press: 21.0 to 21.5 mTorr

[0057] substrate bias: 44 to −35 V

[0058] substrate coolant flow: He, 15 sccm

[0059] plasma confinement current: 0.4 to 0.45 A

[0060] electrode spacing: 6.5 to 6.8 inches

[0061] temperature: 163 EK

[0062] At such settings, the process time for etching may be varied asis appropriate for the desired etch depth and width for a given trench,as well as local and global plasma loading and diffusion effects. Rangesand variations in the above settings enable one to take into accountminor adjustments in the plasma density and substrate bias to allow forthe differences in the open area on the masks. The particular etchprocess parameters set forth above have produced aspect ratios ofgreater than 40:1 at 2 micrometer trench sizes. Furthermore, the aboveparameters have also produced an etch rate of 2.66 micrometers perminute in the minimum capacitive gaps of the MEMS structure illustratedin FIG. 1. The particular exemplary MEMS structure in FIG. 1 is acapacitive rotational accelerometer.

[0063] It is important to note the very low flow rate of the oxygen inthis particular etch system. The low oxygen flow enhances performance ofthe etch system because, generally, the higher the flow of oxygen, thefaster the patterned photoresist mask layer 62 erodes during the etch.Since an oxygen plasma is used specifically for stripping photoresist,the low oxygen flow in the present etch system enables the deep trenches66 to be etched into the bare sections 64 of the epitaxial layer 60before the remaining patterned photoresist is eroded away. In this way,the deep trenches 66 can be etched into the epitaxial layer 60 by solelyutilizing the patterned photoresist as an etch mask instead of alsoutilizing an inorganic hard mask. By solely utilizing the patternedphotoresist as an etch mask, the additional process steps whichtypically accompany utilization of an inorganic hard mask are therebyeliminated, thus saving processing time and expense. Thus, there is asignificant processing advantage to using standard photoresist as theonly etch mask.

[0064] Furthermore, given that thermal expansion mismatch between theepitaxial layer 60 and the photoresist limits the thickness of thephotoresist that can be applied over the epitaxial layer 60, and giventhat thicker photoresist layers craze at higher temperatures thanthinner photoresist layers, it is important to use an etch process withhigh selectivity to the photoresist so that a thin layer of photoresist,which will not craze at room or cryogenic temperatures, may be used tomask the etch without eroding away before the deep trenches 66 breachthe cavity 52 underneath the epitaxial layer 60. In light of such, roomtemperature etch processes (that is, pulsed halogen and carboncompound-forming gas processes) can be utilized as well. In this type ofetch process, the epitaxial layer 60 is bathed with a halogen-containingplasma and a carbon compound-forming gas by any conventional means knownin the art.

[0065] Some processes of the present invention are referred to as beingconducted at room temperature. Room temperature suitable for theseprocesses is in a range of 273 EK±25 EK.

[0066] The oxygen in this preferred etch system functions as apassivation gas on the sidewalls of the trenches 66. As etchingproceeds, the oxygen reacts with the exposed silicon sidewalls to formsilicon dioxide. Because of the highly directional physical component ofthe etch, and the etch selectivity of silicon to silicon dioxide of over150:1 (on a horizontal surface—it is higher still on a verticalsurface), the resulting thin oxide layer on the sidewalls of thetrenches suffices as a passivation layer for the sidewall so that theetch through the epitaxial layer 60 remains anisotropic in nature.Because the oxygen flow is so low, the silicon etch rate is enhanced,leading to high aspect ratios of over 40:1. Since the oxidation reactionof exposed silicon on the bottom of the deepening trench competes withthe chemical and physical components of the silicon reaction with theetchant gas, a lower flow of oxygen favors the etch reaction, resultingin an enhanced etch rate.

[0067] Also, with this particular etch system, a properly micromachinedMEMS structure that is released over the cavity 52 does not exhibitetch-related “stiction.” Stiction is a condition where smooth surfacestend to adhere. Thus, when a micromachined MEMS structure functions inpart by requiring displacement of its projections relative to the firstsubstrate 50 in response to some stimuli, or requires a standoff betweenportions of the projections and the first substrate 50, or must not beelectrically shorted, for example, stiction will render such amicromachined MEMS structure useless. Thus, the fact that this preferredetch method is stiction-free is a significant processing benefit.

[0068] Finally, when etching is completed, remaining areas of thephotoresist layer 62 are then removed from the epitaxial layer 60 andthe MEMS structure 70, as illustrated in FIG. 4(M). This resulting MEMSstructure 70 in FIG. 4(M), thus, represents the body 10 in FIG. 1(without showing such detailed structures such as the cantilevers 18,the projections 20, and the projections 30). Photoresist removal can beaccomplished by any known conventional means, such as, for example, byutilizing conventional liquid resist strippers which cause the remainingphotoresist to swell and lose adhesion to an underlying layer, in thiscase, the epitaxial layer 60. Dry processing (sometimes referred to as“resist ashing”) or plasma ashing (a chemical reaction of O-radicalswith carbon) is preferably used to remove the photoresist by oxidizing(that is, ashing) the photoresist in an oxygen plasma system.

[0069] In light of the above, it is important to note that theelectrically active elements and/or features (such as the body 10, theprojections 20, the projections 30, and the cantilevers 18 of FIG. 1) ofthe MEMS structure 70, fabricated according to the present invention,are substantially contained within a single plane corresponding to thesingle-crystal, epitaxial layer 60. The advantages of such are numerous,as set forth hereinbelow.

[0070] For example, potentially damaging abrasion of the electricallyactive elements and/or features is largely avoided. Also, subsequentencapsulation of the MEMS structure 70 within a “microshell” (that is, acap) is better facilitated, for the microshell can have a thicker roofheight (the portion of the microshell that stands out over the activefeatures/elements must generally be recessed into the top cap to assureno physical hindrance of rotational translation of the activefeatures/elements). That is, in general, the thicker the roof of the topcap, the less sensitive the MEMS structure 70 is to breakage duringpackaging (for example, overmolding).

[0071] Furthermore, utilizing a single-crystal epitaxial layer 60, asopposed to utilizing a thick polysilicon film, is advantageous in thatthe inherent stress in a polysilicon film is much harder to control thanin a single-crystal epitaxial layer. For example, undesired stress,which is somewhat common in thick polysilicon films, can cause elasticdeformation of the surface of the active elements/features or, in theworst case, can cause delamination of the film, thereby leading toimprecise gap sizes after etching. In addition, utilizing asubstantially singleplane, single-crystal epitaxial layer avoids many ofthe problems typically associated with thermal expansion (such asundesirable changes in gap sizes and undesirable changes in capacitancesbetween features separated by such gaps), and enables the simultaneousetching and delineation of both the active features/elements (includingthe body 10, the projections 20, the projections 30, and the cantilevers18 of FIG. 1) as well as any trenches which serve to electricallyisolate active features/elements, metal lines/runners, and pads, asnecessary, for proper electrical communication between the MEMSstructure and any complementary circuitry or electronics. Furthermore,given that such simultaneous etching is performed late in thefabrication process according to the present invention, there is, as aresult, desirably little time for debris to get into the trenches(formed in the epitaxial layer). In this way, debris is effectivelyprevented from hindering rotation of the MEMS structure, prevented fromelectrically shorting active features/elements of the MEMS structure,and prevented from electrically shorting any nearby complementarycircuitry or electronics.

[0072] In addition to the unique implementation of the bond/etch-backmethod set forth hereinabove, it is important to note that there areseveral other general methods of forming a MEMS structure like the onedescribed hereinabove. These other methods include, but are not limitedto, the SOI (silicon on insulator) method, the SOS (silicon on sapphire)method, the silicon with a buried sacrificial layer method, and othersas are known by those skilled in the art. These other methods generallyrequire a vertical etch followed by a lateral etch to release the activefeatures/elements of the MEMS structure. The bond/etch-back method, asdescribed hereinabove, however, requires no lateral etch to define andrelease the MEMS structure.

[0073] Such lateral release etches as are utilized in these othermethods can include, but are not limited to, through-the-wafer etches;selective wet etches of an underlying layer (often an insulator, such asa buried oxide layer, which is common in SO and SOS methods); a dry etchof an underlying layer (such as, for examples, a vapor phasehydrofluoric acid etch of a buried oxide, or a selective lateral etch ofa buried layer in silicon, where the etchant species attacks asub-surface layer selectively thereby freeing the MEMS structure fromthe substrate); and a set of methods where, after the vertical etch iscomplete, a nonselective lateral etch is performed at the bottom of thevertical etch thereby freeing the MEMS structure. These other methodsand their lateral release etches typically have limited lateral etchrates. Thus, to effectively utilize these methods in a manufacturingprocess, the lateral distances to be etched must necessarily be small.Such generally dictates that any large areas that need to be released,such as body masses, must be perforated at regular intervals to allowthe lateral etch to free the area (for example, body mass) in anacceptable amount of time. The bond/etch-back method, in contrast, doesnot require such perforations. As a result, utilizing the bond/etch-backmethod, as uniquely implemented hereinabove, is the preferred method forfabricating MEMS structures, for it offers process advantages and savescost. In particular, the lack of utilizing perforations in thebond/etch-back method allows for uni-body construction of the body mass,allows for a larger overall body mass, and thus allows for a largerrotational inertia for a same-sized body mass.

[0074] Although utilizing the bond/etch-back method as uniquely tailoredhereinabove according to the present invention is preferred, such othergeneral methods of fabricating a MEMS device can also be uniquelytailored to fabricate a MEMS structure with interdigitated projections.However, it is to be understood that the step of etching throughsections of the epitaxial layer may require both vertical and lateraletching as dictated by the above-mentioned other general methods offabricating a MEMS structure (that is, the methods other than thebond/etch-back method).

[0075] While the present invention has been described in what ispresently considered to be the most practical and preferred embodimentand/or implementation, it is to be understood that the invention is notto be limited to the disclosed embodiment, but on the contrary, isintended to cover various modifications and equivalent arrangementsincluded within the spirit and scope of the appended claims, which scopeis to be accorded the broadest interpretation so as to encompass allsuch modifications and equivalent structures as is permitted under thelaw.

1. A MEMS sensor structure of the type comprising the uniplanarcombination of a body defining a proof mass supported for compliance ina predetermined mode by one or more integral suspension projectionswhich extend from said body to a support member wherein the improvementcomprises: a first plurality of rigid sensory projections between eachof said suspension projections extending from said body toward butspaced from said support member; and a second plurality of complementaland electrically isolated sensory projections extending from saidsupport member toward but spaced from said body; said first and secondpluralities of projections being arranged in complemental pairs anddefining on one side of each of said projections a primary sensing gapof essentially uniform width over the entire longitudinal extentthereof.
 2. A sensory structure as defined in claim 1 wherein the proofmass is essentially circular, the projections are radial and thecompliance mode is rotary.
 3. A sensor as defined in claim 1 wherein thefirst and second projections have rounded tips which form contiguousportions of the essentially uniform width primary sensing gap.
 4. Asensor structure as defined in claim 2 wherein the second plurality ofsensory projections are integrally grouped and joined with arcuateperipheral portions overlying said support plane, said suspensionprojection lying between said groups and spaced therefrom by a gap whichis not less than said primary sensing gap.
 5. A sensor structure asdefined in claim 2 wherein the first plurality of sensory projectionsare integrally grouped and joined with arcuate portions of said proofmass overlying said support plane, said suspension projection lyingbetween said groups and spaced therefrom by a gap which is not less thansaid primary sensing gap.
 6. A sensor structure for amicro-electro-mechanical rotary accelerometer of the type comprising theuniplanar combination of a generally circular body and, integraltherewith, a plurality of radially outwardly extending projectionsarranged in complemental pairs with electrically isolated inwardlyextending projections characterized by arranging some of said radialextensions to operate only as suspension elements which are coupled withthe inwardly extending projections by capacitive couplings which areweak relative to the capacitive coupling of said complemental pairs. 7.A sensor structure as defined in claim 6 wherein the complemental pairsare coupled by uniform capacitive gaps.
 8. A micro-electro-mechanicalstructure comprising: a semiconductor layer defining a body (10) and arim structure (16) surrounding said body; an underlying support meansfor said rim structure; a first plurality of flexible projections (18)extending from said body to said support means; a second plurality ofprojections (20) extending integrally from said body toward but spacedfrom said rim structure; each said second projection having a baseintegral with said body and a tip proximate but spaced from said rim;and a third plurality of projections (30) extending from said rimstructure substantially toward but spaced from said body, each suchthird projection having a base attached to said rim, and a tip proximatesaid body; wherein said second projections (20) and said thirdprojections (30) are arranged in a substantially interdigitated fashionsuch that each said second projection is positioned proximate to adifferent one of said third projections to thereby define a plurality ofproximate projection pairs, such that a minimum capacitive gap issubstantially defined between said second projection and said thirdprojection of each said projection pair, with each said projection pairbeing separated from any adjacent projection pair by a gap greater thansaid primary capacitive gap, such that each said tip of each of saidsecond projections is spaced from said rim by a distance substantiallyequal to said primary capacitive gap, and such that each said tip ofeach of said third projections is spaced from said body by a distancesubstantially equal to said primary capacitive gap.
 9. Themicro-electro-mechanical structure according to claim 8, wherein saidrim is shaped such that the circumference of said tip of each saidsecond projection is substantially uniformly spaced from said rim by adistance substantially equal to said primary capacitive gap.
 10. Themicro-electro-mechanical structure according to claim 8, wherein saidbody is shaped such that the circumference of said tip of each saidthird projection is substantially uniformly spaced from said body by adistance substantially equal to said primary capacitive gap.
 11. Themicro-electro-mechanical structure according to claim 8, wherein eachsaid first projection, said semiconductor layer, said body, said secondprojections, and said third projections are comprised of an electricallyconductive, doped semiconductor material such that capacitance iscapable of being electrically measured between said second projectionsand said third projections.
 12. The micro-electro-mechanical structureaccording to claim 11, wherein said electrically conductive, dopedsemiconductor material is p-doped, epitaxial silicon.
 13. Themicro-electro-mechanical structure according to claim 8, wherein eachsecond projection is substantially straight and is substantially uniformin width along the length of said first projection.
 14. Themicro-electro-mechanical structure according to claim 13, wherein oneside of said second projection and one side of said third projection ofeach said projection pair is substantially uniformly spaced apart fromeach other, along said length of said second projection, by a distancesubstantially equal to said primary capacitive gap.
 15. Themicro-electro-mechanical structure according to claim 8, said structurefurther comprising: a substrate having a surface and a cavity definedwithin said surface; and an isolation layer on top of said surface ofsaid substrate and the lining of said cavity, wherein said semiconductorlayer is on top of said isolation layer such that said body, said secondprojections, and said third projections are suspended over said cavity.16. The micro-electro-mechanical structure according to claim 15,wherein said isolation layer comprises silicon dioxide.
 17. A processfor fabricating a micro-electro-mechanical structure havinginterdigitated projections, said process comprising the steps of:providing a first substrate; etching a cavity within said firstsubstrate; forming an isolation layer on said first substrate; providinga second substrate; doping the top portion of said second substrate tothereby form an etch termination layer; forming a doped epitaxial layeron the etch termination layer portion of said second substrate such thatsaid etch termination layer portion of said second substrate has ahigher doping concentration than said epitaxial layer; bonding saidsecond substrate to said first substrate such that said epitaxial layercovers said cavity and is bonded to said isolation layer at theperiphery of said cavity; removing the non-termination layer portion ofsaid second substrate from said etch termination layer portion of saidsecond substrate; removing said etch termination layer portion of saidsecond substrate from said epitaxial layer; applying photoresist on saidepitaxial layer; patterning said photoresist according to apredetermined shape of said micro-electro-mechanical structure;anisotropically etching through sections of said epitaxial layerdisposed over said cavity and revealed through said patternedphotoresist to thereby define and release said micro-electro-mechanicalstructure above said cavity; and removing said patterned photoresist.18. The process according to claim 17, wherein said top portion of saidsecond substrate is doped with a p-type dopant comprising boron andgermanium.
 19. The process according to claim 17, wherein said epitaxiallayer is doped with a p-type dopant.
 20. The process according to claim17, wherein the photoresist is a positive photoresist.
 21. The processaccording to claim 17, wherein the step of anisotropically etchingthrough said epitaxial layer is accomplished by contacting saidepitaxial layer with a plasma comprising sulfur hexafluoride and oxygen.22. The process according to claim 21, wherein the step ofanisotropically etching through said epitaxial layer includes coolingthe epitaxial layer to a cryogenic temperature of less than about 173EK.
 23. The process according to claim 17, wherein the step ofanisotropically etching through said epitaxial layer is accomplished bya pulsed halogen and carbon compound-forming gas process.
 24. Theprocess according to claim 23, wherein the step of anisotropicallyetching through said epitaxial layer is performed at room temperature.25. The process according to claim 17, wherein said first substrate andsaid second substrate comprise silicon, and wherein said isolation layercomprises silicon dioxide.
 26. The process according to claim 17,wherein the step of patterning said photoresist according to apredetermined shape of said micro-electromechanical structure includesthe steps of: determining a minimum capacitive gap between saidinterdigitated projections of said micro-electro-mechanical structurewhich are nearest to each other, wherein each projection of saidinterdigitated projections has a base and a tip at opposite ends;defining said predetermined shape such that each base of each saidprojection of said interdigitated projections is proximate to at leastone tip of another said projection of said interdigitated projections bya distance substantially equal to said minimum capacitive gap; andselectively removing said photoresist to reveal bare sections of saidepitaxial layer according to said predetermined shape.